Add Vsrl and Vsra (shift right) instructions

Relies on compiler-defined right shift behavior based on unsigned and
signed integral types, corresponding to logical and arithmetic right
shifts, respectively.

Tests rely on custom sources to ensure most significant bit is set.
These inputs properly test differences between logical and arithmetic
right shifts.

Also renames MaskBits helper to MaskBitsForShift to clarify intention.

Bug: 316602047
Test: berberis_host_tests
Change-Id: I1d099b664d4f9a4bb263f40ccdd1409306bf1452
3 files changed